Logic Gates Questions and Answers

Answer: B

The symbol has a curved back edge and a pointed front, and the output has no small circle (bubble) on it. That is the OR gate symbol: the output is high when either input is high. An AND gate has a flat back, a NAND gate is an AND with a bubble, and a NOT gate is a triangle with a bubble, drawn with one input. The answer is OR, option B.

Answer: D

The gate gives a high output only when exactly one of the two inputs is high, and a low output when both are low and also when both are high. That is the exclusive OR (XOR) gate, with the truth table 00 gives 0, 01 gives 1, 10 gives 1, 11 gives 0.

An ordinary OR gate gives a high output also when both inputs are high, so it does not fit the word 'only'. A NAND gate gives low only when both inputs are high, and a NOR gate gives high only when both are low. The answer is XOR, option D.

Answer: A

Trace the two branches of the diagram with \(A = 1\) and \(B = 0\). The top AND gate has the inputs A and B directly, so its output is \(A \cdot B = 1 \cdot 0 = 0\).

The bottom AND gate gets the inputs through the NOT gates, so its inputs are \(\bar{A} = 0\) and \(\bar{B} = 1\), and its output is \(0 \cdot 1 = 0\).

\[ Y = 0 + 0 = 0 \]

The OR gate gives 0 only when both of its inputs are 0, which is the case. Options 2 and 3 are not possible as the output of a logic gate, which is only 0 or 1, and 1 would need one of the AND outputs to be 1. The answer is 0, option A.